1. Field of the Invention
The present invention relates to a tensioner that provides appropriate tension to a running chain, belt, or the like.
2. Description of the Related Art
It has been common practice to use a tensioner in order to maintain appropriate tension in a chain or the like. For example, a known chain guide mechanism, which uses a tensioner lever to slidably guide a transmission chain such as an endless roller chain passing over respective sprockets of a crankshaft and a casts shaft inside an engine room, biases the tensioner lever by using a tensioner in order to maintain appropriate tension of the chain or the like.
The known tensioner 510 used in such a chain guide mechanism includes, for example, as shown schematically in FIG. 10, a tensioner body 520 having a cylindrical body hole 521 with an open end, a plunger slidably inserted in the cylindrical body hole 521, and biasing unit 560 for biasing the plunger 540 toward a front side (see, for example, Japanese Patent Application Laid-open No. 2009-002495).
In this tensioner 510, oil is supplied from an oil supply hole 523 formed in the tensioner body 520, so that an oil pressure chamber 511 forced between the cylindrical body hole 521 and the plunger 540 is filled with the oil. This oil biases the plunger 540 toward the front side. A check valve 550 (schematically shown as a check ball) stops the oil from flowing oat from the oil supply hole 523. As the plunger 540 reciprocates, the oil flows through the small gap between the plunger 540 and the cylindrical body hole 521, and the flow resistance provides the damping effect of slowing down the reciprocal movement of the plunger 540.
One problem with the tensioner 510 described above is that, since the tensioner body 520 is a die-cast product made of cast iron or aluminum alloy, the cylindrical body hole 521 in which the plunger 540 slides needs to be subjected to film formation by coating, or smoothing, or machining and the like, in order to achieve surface precision and durability for the purpose of preventing wear and seizure of the plunger 540.
One measure that has been proposed to be taken when the surface precision and durability of the cylindrical body hole 521 are not satisfactory is to interpose a separate component, such as a metal sleeve 530 shown in FIG. 11, between the inner circumferential surface of the cylindrical body hole 521 and the outer circumferential surface of the plunger 540 so as to allow selection of materials for the tensioner body 520 and to achieve a certain degree of design freedom with respect to the treatment of the inner circumferential surface of the cylindrical body hole 521 (see, for example, Japanese latent Application. Laid-open No. 2002-206603).